Computed tomography (CT) provides a fast, noninvasive technique for obtaining slice images of the coronary arteries, from which the location and the extent of calcium deposits are evident. Commercially available data-processing programs afford the possibility of determining the proportion of calcium in the coronary arteries on the basis of the 3D image data or volume data from computed tomography. For this, a native CT scan is required initially, i.e. a record of 3D image data of the heart without a contrast agent. The user must then manually mark regions identified as calcium deposits in the various displays, which are later intended to contribute to quantifying the proportion of calcium. In general, the different coronary arteries are characterized by the user for this and labeled accordingly, for example using the conventional descriptors LM (left main), LAD (left anterior descending), LCX (left circumflexus) and RCA (right coronary artery). The proportions of calcium are then calculated in each case for the individual coronary arteries using the volumes of the marked regions.
The calculation is generally performed as a weighted sum of all pixels or voxels in the marked regions, which pixels or voxels lie above a threshold for the HU value, typically 130 HU. In the process, the weighting factor depends in each case on the maximum HU value within the region. This technique of weighting was originally introduced by Agatston and so the proportion of calcium determined thereby is also referred to as the Agatston score. After the native CT scan, a further CT scan may be performed under the administration of a contrast agent in such examinations in order to obtain a CT angiography record for a display with a high contrast.
Determining the proportion of calcium has previously been very complicated because the user had to view all individual images, identify the calcium deposits and mark them accordingly. On the one hand, this takes up a lot of time and, on the other hand, the reproducibility of the results depends strongly on the user. As a result of the two CT scans, the radiation dose to which the patient is exposed also increases.